Exhaust purifying apparatus and exhaust purifying method for internal combustion engine

ABSTRACT

An exhaust purifying apparatus estimates an accumulation amount of particulate matter trapped about a catalyst in an exhaust system. When the estimated accumulation amount is equal to or more than a permissible value, the apparatus executes PM elimination control for supplying unburned fuel component to the catalyst. The apparatus sets the estimated accumulation amount to zero at the completion of the PM elimination control. When execution of the PM elimination control becomes possible after suspension of the control, the apparatus resumes the PM elimination control even if the accumulation amount is less than the permissible value. Therefore, The estimated accumulation amount is prevented from being significantly deviated from the actual accumulation amount due to suspension.

TECHNICAL FIELD

The present invention relates to an exhaust purifying apparatus and anexhaust purifying method for an internal combustion engine.

BACKGROUND ART

As disclosed in Japanese Laid-Open Patent Publication No. 5-44434, atypical exhaust purifying apparatus applied to an internal combustionengine such as a vehicle diesel engine includes a PM filter that islocated in the exhaust system. The PM filter traps particulate matter,which is predominantly composed of soot in exhaust gas.

In an internal combustion engine provided with such an exhaust purifyingapparatus, PM elimination control is performed to prevent the PM filterfrom being clogged with particulate matter (PM) when the accumulationamount of particulate matter in the PM filter estimated from, forexample, the operating condition of the engine is more than or equal toa permissible value. In the PM elimination control, fuel is added toexhaust gas in a section upstream of the PM filter so that oxidation ofunburned fuel component on the catalyst of the PM filter generates heatto increase the temperature of the catalyst. Accordingly, particulatematter on the PM filter is burned. When it is determined thatparticulate matter deposited on the PM filter is completely burned, theestimated accumulation amount of particulate matter on the PM filter isset to zero, and the PM elimination control is completed.

The PM elimination control is sometimes suspended due to stopping of theengine during the execution. When the engine is started again andexecution of the PM elimination control becomes possible, the PMelimination control is not resumed if the accumulation amount ofparticulate matter at the time of suspension is less than thepermissible value. However, if incomplete execution of the PMelimination control due to suspension is repeated several times, thefollowing drawbacks are likely to occur in relation to the estimatedaccumulation amount of particulate matter.

The estimated accumulation amount of particulate matter may contain anerror in relation to the actual accumulation amount. Such an error iseliminated by setting the estimated amount of particulate matter to zerowhen the PM elimination control is completed so that particulate matterdeposited on the PM filter is completely burned. However, afterexecution and suspension of the PM elimination control are repeated afew times, accumulation of particulate matter on the PM filter in normaloperation of the engine and burning of the particulate matter in the PMelimination control up to its suspension are repeated without theestimated particulate matter accumulation amount being set to zero.While the accumulation amount of particulate matter is repeatedlyincreased and reduced, the estimated accumulation amount can be greatlydeviated from the actual accumulation amount.

When the estimated accumulation amount of particulate matter issignificantly less than the actual accumulation amount, execution ofcontrol for richening the exhaust air-fuel ratio based on the estimatedaccumulation amount of particulate matter can excessively increase thecatalyst bed temperature of the PM filter. Such an excessive catalystbed temperature is caused in the following manner. When unburned fuelcomponent is supplied to the PM filter based on the control forrichening the exhaust air-fuel ratio, oxidation of the unburned fuelcomponent causes particulate matter deposited on the PM filter to burn.At this time, a greater amount of particulate matter than estimated hasbeen accumulated, and the heat generated when the particulate matter isburned is increased accordingly.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide anexhaust purifying apparatus of an internal combustion engine thatprevents an estimated accumulation amount of particulate matter about acatalyst from being significantly deviated from the actual accumulationamount due to suspension of PM elimination control. The presentinvention further provides an exhaust purifying method.

To achieve the foregoing and other objectives and in accordance with thepurpose of the present invention, the invention provides an exhaustpurifying apparatus for an internal combustion engine. The apparatusestimates an accumulation amount of particulate matter trapped about acatalyst in an exhaust system. When the estimated accumulation amount isequal to or more than a permissible value, the apparatus executes PMelimination control for supplying unburned fuel component to thecatalyst to increase the temperature of the catalyst and burning thetrapped particulate matter. The apparatus sets the estimatedaccumulation amount to zero at the completion of the PM eliminationcontrol. When execution of the PM elimination control becomes possibleafter suspension of the control, the apparatus resumes the PMelimination control even if the accumulation amount of particulatematter about the catalyst is less than the permissible value.

The present invention further provides an exhaust purifying method foran internal combustion engine. The method includes estimating anaccumulation amount of particulate matter trapped about a catalyst in anexhaust system of the internal combustion engine. The method furtherincludes executing PM elimination control when the estimatedaccumulation amount is equal to or more than a permissible value. Inwhich control, unburned fuel component is supplied to the catalyst toincrease the temperature of the catalyst and the trapped particulatematter is burned. The method further includes setting the estimatedaccumulation amount to zero at the completion of the PM eliminationcontrol. The method further includes resuming the PM elimination controlwhen execution of the PM elimination control becomes possible aftersuspension of the control, even if the accumulation amount ofparticulate matter about the catalyst is less than the permissiblevalue.

Other aspects and advantages of the invention will become apparent fromthe following description, taken in conjunction with the accompanyingdrawings, illustrating by way of example the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with objects and advantages thereof, may best beunderstood by reference to the following description of the presentlypreferred embodiments together with the accompanying drawings in which:

FIG. 1 is a diagrammatic view illustrating the overall configuration ofan internal combustion engine to which an exhaust purifying apparatusaccording to the present invention is applied;

FIG. 2 is a graph showing changes in combustion rate of unburned fuel(HC) and PM about a catalyst relative to changes in the catalyst bedtemperature;

FIGS. 3(a) and 3(b) are time charts illustrating changes in PMaccumulation amount and catalyst bed temperature during PM eliminationcontrol;

FIG. 4 is a graph showing the relationship between a PM accumulationamount (determination value) Dc and holding period t2;

FIG. 5 is a graph showing the relationship between a PM accumulationamount (determination value) Db and holding period t3;

FIGS. 6(a) and 6(b) are time charts illustrating the manner of addingfuel and changes in the exhaust air-fuel ratio due to the fuel additionduring burn-up control;

FIGS. 7(a) and 7(b) are time charts illustrating changes in the PMaccumulation amount and the catalyst bed temperature when the PMelimination control is executed and completed with the estimated PMaccumulation amount being deviated from the actual accumulation amount;

FIGS. 8(a) and 8(b) are time charts illustrating changes in the PMaccumulation amount and the catalyst bed temperature when the PMelimination control is repeatedly executed and suspended;

FIGS. 9(a) and 9(b) are time charts illustrating changes in the PMaccumulation amount and the catalyst bed temperature when the PMelimination control is first suspended and then resumed and completed;and

FIG. 10 is a flowchart showing the procedure for resuming the PMelimination control.

BEST MODE FOR CARRYING OUT THE INVENTION

An exhaust purifying apparatus for an internal combustion engineaccording to a preferred embodiment of the present invention will now bedescribed with reference to FIGS. 1 to 10(d).

FIG. 1 illustrates the configuration of an internal combustion engine 10to which the exhaust purifying apparatus according to this embodiment isapplied. The internal combustion engine 10 is a diesel engine thatincludes a common rail fuel injection device, and a turbocharger 11. Theengine 10 includes an intake passage 12, combustion chambers 13, and anexhaust passage 14.

The intake passage 12 forms an intake system for the internal combustionengine 10. In the most upstream section of the intake passage 12, an aircleaner 15 is located. From the air cleaner 15 toward the downstreamside, the air flow meter 16, a compressor 17 incorporated in theturbocharger 11, an intercooler 18, and an intake throttle valve 19 areprovided in the intake passage 12. The intake passage 12 is branched atan intake manifold 20 located downstream of the intake throttle valve19, and connected to each of the combustion chambers 13 of the internalcombustion engine 10 through intake ports 21.

In the exhaust passage 14, which forms part of the exhaust system forthe internal combustion engine 10, an exhaust port 22 is connected toeach combustion chamber 13. The exhaust ports 22 are connected to anexhaust turbine 24 of the turbocharger 11 through an exhaust manifold23. In a section of the exhaust passage 14 that is downstream of theexhaust turbine 24, a NOx catalytic converter 25, a PM filter 26, and anoxidation catalytic converter 27 are provided in this order from theupstream side.

The NOx catalytic converter 25 supports an occlusion-reduction NOxcatalyst. The NOx catalyst occludes NOx in exhaust gas when theconcentration of oxygen in exhaust gas is high, and emits the occludedNOx when the concentration of oxygen in the exhaust gas is low. If asufficient amount of unburned fuel component, which functions as areducing agent, exists in the vicinity thereof, the NOx catalyst reducesemitted NOx to purify the exhaust gas.

The PM filter 26 is made of a porous material and traps particulatematter (PM), which is predominantly composed of soot, in exhaust. Likethe NOx catalytic converter 25, the PM filter 26 supports anocclusion-reduction NOx catalyst. The NOx catalyst of the PM filter 26reduces emitted NOx to purify the exhaust gas. The reaction triggered bythe NOx catalyst burns (oxidizes) and removes the trapped PM.

The oxidation catalytic converter 27 supports an oxidation catalyst. Theoxidation catalyst oxidizes hydrocarbon (HC) and carbon monoxide (CO) inexhaust gas to purify the exhaust gas.

In sections upstream of and downstream of the PM filter 26, an input gastemperature sensor 28 and an output gas temperature sensor 29 areprovided, respectively. The input gas temperature sensor 28 detects aninput gas temperature, which is the temperature of exhaust gas thatflows into the PM filter 26. The output gas temperature sensor 29detects an output gas temperature, which is the temperature of exhaustgas that has passed through the PM filter 26. Also, a differentialpressure sensor 30 is provided in the exhaust passage 14. Thedifferential pressure sensor 30 detects a pressure difference between asection upstream and a section downstream of the PM filter 26. Oxygensensors 31, 32 are located in a section of the exhaust passage 14 thatis upstream of the NOx catalytic converter 25 and a section of theexhaust passage 14 between the PM filter 26 and the oxidation catalyticconverter 27, respectively. The oxygen sensors 31, 32 detect theconcentration of oxygen in exhaust gas.

The internal combustion engine 10 further includes an exhaust gasrecirculation device (EGR device) for returning some of the exhaust gasto the air in the intake passage 12. The EGR device includes an EGRpassage 33 that connects the exhaust passage 14 with the intake passage12. The most upstream section of the EGR passage 33 is connected to asection of the exhaust passage 14 that is upstream of the exhaustturbine 24.

In the EGR passage 33, an EGR catalyst 34, an EGR cooler 35, and an EGRvalve 36 are provided in this order from the upstream side. The EGRcatalyst 34 reforms recirculated exhaust gas. The EGR cooler 35 coolsthe reformed exhaust gas. The EGR valve 36 adjusts the flow rate of thereformed and cooled exhaust gas. The most downstream section of the EGRpassage 33 is connected to a section of the intake passage 12 that isdownstream of the intake throttle valve 19.

An injector 40 is provided in each combustion chamber 13 of the internalcombustion engine 10 to inject fuel to be combusted in the combustionchamber 13. The injectors 40 are connected to a common rail 42 with ahigh-pressure fuel pipe 41. High-pressure fuel is supplied to the commonrail 42 through a fuel pump 43. The pressure of high-pressure fuel inthe common rail 42 is detected by a rail pressure sensor 44 attached tothe common rail 42. The fuel pump 43 is capable of supplyinglow-pressure fuel to a fuel adding valve 46 through a low-pressure fuelpipe 45.

Various control procedures for the internal combustion engine 10 areexecuted by an electronic control device 50. The electronic controldevice 50 includes a CPU that executes various computation processesrelated to control of the engine 10, a ROM storing programs and datanecessary for the control, a RAM for temporarily storing the computationresults of the CPU, and input and output ports for inputting andoutputting signals from and to the outside.

In addition to the above described sensors, the input port of theelectronic control device 50 is connected to an NE sensor 51 fordetecting the rotational speed of the engine 10, an acceleration pedalsensor 52 for detecting the degree of depression of an accelerationpedal, and a throttle valve sensor 53 for detecting the opening degreeof the intake throttle valve 19. The output port of the electroniccontrol device 50 is connected to a drive circuit for driving the intakethrottle valve 19, the EGR valve 36, the injector 40, the fuel pump 43,and the fuel adding valve 46.

Based on detected signals from the above described sensors, theelectronic control device 50 grasps the operating condition of theengine 10. According to the grasped operating condition, the electroniccontrol device 50 outputs command signals to the drive circuits of thedevices connected to the output port. The electronic control device 50executes various control procedures such as control of the openingdegree of the intake throttle valve 19, EGR control based on the openingdegree control of the EGR valve 36, control of the amount, the timingand the pressure of fuel injection from the injector 40, and controlrelated to fuel addition by the fuel adding valve 46.

In this embodiment, to prevent the NOx catalytic converter 25 and the PMfilter 26 from being clogged with PM, PM elimination control isperformed, in which PM trapped by the NOx catalytic converter 25 and thePM filter 26 are burned to purify exhaust gas. In the PM eliminationcontrol, unburned fuel component is supplied to the NOx catalyticconverter 25 and the NOx catalyst of the PM filter 26 so that theunburned fuel component is oxidized in exhaust gas or on each catalystto generate heat. Accordingly, the catalyst is heated to a temperatureof about 600 to 700° C., and PM about the catalyst is burned.

During the PM elimination control, unburned fuel component may besupplied to the catalysts by sub-injection (after injection) in anexhaust stroke or an expansion stroke, which injection is executed afterfuel is injected from the injector 40 to be combusted in the combustionchambers 13. Alternatively, unburned fuel may be supplied by adding fuelto exhaust gas from the fuel adding valve 46. To minimize extra fuelconsumption due to the PM elimination control, the amount of unburnedfuel component added to the catalysts in the PM elimination control islimited to the minimum value that allows for a necessary increase in thetemperature of the catalysts.

In this embodiment, the PM elimination control is performed when thefollowing requirements are all satisfied.

The elimination of PM is requested. A request for the PM elimination ismade when clogging of the PM filter 26 is recognized based on the factthat the PM accumulation amount of the PM filter 26 estimated from theengine operating condition reaches and exceeds the highest value of apermissible range.

A detected value of the input gas temperature sensor 28 (input gastemperature thci) is more than or equal to a lower limit temperature A(for example, 150° C.) for performing the PM elimination control. Also,the catalyst bed temperature of the NOx catalyst, which is estimatedfrom the history of the engine operating condition is more than or equalto a lower limit temperature B for performing the PM eliminationcontrol. The lower limit temperatures A, B are the lowest values of theexhaust temperature and the catalyst bed temperature that causeoxidation sufficient to increase the catalyst bed temperature asunburned fuel component is supplied.

The detected value of the input gas temperature sensor 28 is less thanan upper limit value C in a temperature range for avoiding excessivetemperature increase of the catalysts due to heat generated by the PMelimination control.

The detected value of the output gas temperature sensor 29 is less thanan upper limit value D in a temperature range for avoiding excessivetemperature increase of the catalysts due to the PM elimination control.

Fuel addition to exhaust gas is permitted. That is, the engine operatingcondition is in a range to permit the fuel addition to exhaust gas. Theaddition of fuel to exhaust gas is permitted in the internal combustionengine 10 as long as the engine 10 is not stalling, the cylinders havebeen distinguished, and the depression degree of the acceleration pedalis not limited.

The PM elimination control will now be described with reference to FIG.2 to 6(b).

FIG. 2 is a graph showing changes in the combustion rates of unburnedfuel (HC) and PM collected on the surface of the catalysts as thecatalyst bed temperature increases at the NOx catalytic converter 25 andthe PM filter 26. As obvious from FIG. 2, unburned fuel collected on thecatalysts is burned at a relatively low catalyst bed temperature (about300° C.), and PM collected on the catalysts is burned when the catalystbed temperature is increased to a relatively high temperature, forexample, in the range between 600 and 700° C., inclusive. Therefore, ifthe catalyst bed temperature is suddenly increased to a value of about700° C., a great amount of unburned fuel and PM collected on thecatalysts are burned, and the generated heat can excessively increasethe catalyst bed temperature.

Such an excessive increase in the catalyst bed temperature is preventedby discretely increasing the catalyst bed temperature as shown in FIG.3(b). That is, to burn unburned fuel (HC) and PM collected on thesurface of the catalysts in stages, the catalyst bed temperature isincreased to 300, 600, 630, and 650° C., successively. Specifically,first the minimum amount of unburned fuel component required forincreasing the catalyst bed temperature to 300° C. is supplied to thecatalysts. As the catalyst bed temperature is increased toward 300° C.,unburned fuel collected on the catalysts is burned. When the catalystbed temperature reaches 300° C., the catalyst bed temperature isincreased to 600° C. and is held at this temperature for holding periodt2. Then, the catalyst bed temperature is increased to 630° C. and isheld at this temperature for holding period t3. Finally, the catalystbed temperature is increased to 650° C.

As shown in FIG. 4, the holding period t2, during which the catalyst bedtemperature is held at 600° C., is set shorter as the PM accumulationamount (determination value) Dc at time Tc, where the catalyst bedtemperature is switched from 300° C. to 600° C., is reduced. As shown inFIG. 5, the holding period t3, during which the catalyst bed temperatureis held at 630° C., is set shorter as the PM accumulation amount(determination value) Db at time Tb, where the catalyst bed temperatureis switched from 600° C. to 630° C., is reduced. The holding periods t2and t3 are varied according to the PM accumulation amounts Dc, Db sothat the time for the PM elimination control to burn PM is minimized,and deterioration of the fuel consumption due to the amount of fuel usedin the control is minimized.

As the catalyst bed temperature is increased in stages in the abovedescribed PM elimination control, PM collected about the catalysts isburned, and the PM accumulation amount is reduced as shown in FIG. 3(a).However, at the upstream end of the NOx catalytic converter 25 and theupstream end of the PM filter 26, some PM remains even if the abovedescribed PM elimination control is performed. The reason why PM remainsis believed to be that PM is likely to be deposited at the exhaustupstream end of the NOx catalytic converter 25 and the exhaust upstreamend of the PM filter 26, and the supply of unburned fuel component inthe PM elimination control cannot supply a sufficient amount of unburnedfuel component per unit time to burn the PM completely. Particularly, inthe NOx catalytic converter 25, which is located upstream of the PMfilter 26, a greater amount of PM that is not burned in the PMelimination control remains at the upstream end.

Thus, at the final stage of the PM elimination control, that is when thePM accumulation amount is reduced to a determination value Da close tozero (for example, 0.3 g), burn-up control is performed to burn PM thatcannot be burned in the PM elimination control. The overview of theburn-up control will be described with reference to FIGS. 6(a) and 6(b).FIG. 6(a) shows the manner in which the fuel adding valve 46 adds fuel,and FIG. 6(b) shows changes in the exhaust air-fuel ratio.

As shown in FIG. 6(a), concentrated intermittent fuel addition isrepeatedly performed and stopped in the burn-up control. Theconcentrated intermittent fuel addition increases the amount of unburnedfuel component and oxygen supplied to the catalysts of the NOx catalyticconverter 25 and the PM filter 26 to a level sufficient for burning thePM that cannot be burned in the PM elimination control. Therefore, theconcentrated intermittent fuel addition permits the PM to be burned.

The concentrated intermittent fuel addition unavoidably causes thecatalyst bed temperature to increase noticeably. Thus, the fuel additionis periodically stopped, thereby suppressing excessive increase in thecatalyst bed temperature. As a result, intermittent concentrated fueladdition is repeatedly performed and stopped, and the exhaust air-fuelratio is repeatedly reversed between a rich state and a lean state asshown in FIG. 6(b). The burn-up control is ended when the repetitions ofperforming and stopping of the concentrated intermittent fuel additionhas reached a number (in this embodiment, three times) that issufficient for burning the PM remaining in the NOx catalytic converter25 and the PM filter 26.

The PM elimination control is completed based on the end of the burn-upcontrol. When the PM elimination control is completed, the PMaccumulation amount about the catalysts estimated from the engineoperating condition becomes zero. In other words, the PM accumulationamount is set to zero when the PM elimination control is completed.

The PM elimination control may be suspended during execution. Forexample, when the engine 10 is stopped, the PM elimination control issuspended even during the execution thereof. Also, the PM eliminationcontrol is suspended when deactivation of the catalyst occurs, in whichthe catalyst bed temperature is lowered even if unburned fuel componentis being supplied, due to a drop of the exhaust temperature.

Such deactivation of a catalyst is caused by a vicious circle in which adrop in the exhaust temperature during the PM elimination controldeactivates a catalyst and temporarily hampers oxidation of unburnedfuel, and the unburned fuel stays collected on the catalyst anddecreases the surface area of the catalyst that is exposed to exhaustgas, and the degree of activation of the catalyst is lowered further,and so on. If unburned fuel component is supplied to each catalyst inthe PM elimination control during deactivation of the catalyst, theunburned fuel component is emitted to the outside in an incompletecombustion state. Therefore, the exhaust emission can be deteriorated.For example, a great amount of smoke may be emitted. The PM eliminationcontrol is thus suspended when the catalyst is deactivated.

However, if incomplete execution of the PM elimination control due tosuspension is repeated several times, the estimated PM accumulationamount is greatly deviated from the actual accumulation amount, whichcauses problems. The reason why repetitive execution and suspension ofthe PM elimination control causes the estimated PM accumulation amountto be deviated from the actual accumulation amount will be describedwith reference to FIGS. 7(a) to 8(b).

Since the PM accumulation amount used in the PM elimination control is avalue estimated from the engine operating condition, the PM accumulationamount can be deviated from the actual accumulation amount. For example,as shown in FIG. 7(a), the estimated PM accumulation amount (solid lineL1) can be deviated from the actual accumulation amount (broken lineL2). Normally, such an error is eliminated by setting the estimated PMaccumulation to zero when PM collected about the catalyst is completelyburned and the PM elimination control is completed. That is, as shown inFIG. 7(b), when the PM elimination control that has been started at timeT1, where the estimated PM accumulation amount reaches and exceeds apermissible value, is completed at time T2, the PM collected about eachcatalyst is completely burned, and the estimated PM accumulation amountis set to zero. This allows the estimated PM accumulation amount tocorrespond to the actual accumulation amount, and an error between thesevalues is eliminated.

As described above, if the PM elimination control is completed, an errorbetween the estimated PM accumulation amount and the actual accumulationamount is eliminated. However, if the PM elimination control issuspended before it is completed, such an error is not eliminated. Forexample, as shown in FIG. 8(a), when the PM elimination control issuspended at time T3 due to stopping of the engine 10 or deactivation ofthe catalysts, if the PM accumulation amount has dropped to a value lessthan the permissible value at the time of suspension, the PM eliminationcontrol will not be resumed even if the engine 10 is started again orthe catalyst is activated so that the PM elimination control ispossible. In this case, since the PM elimination control is notcompleted, setting the estimated PM accumulation amount to zero toeliminate the error is not performed. Then, when the PM accumulationamount reaches the permissible value again, the PM elimination controlis executed (time T4).

If execution and suspension of the PM elimination control are repeated afew times (T4 to T7 in FIG. 8(b)), accumulation of PM about eachcatalyst in a normal operation of the engine 10 and burning of the PM inthe PM elimination control are repeated without the PM accumulationamount being set to zero. As a result, the estimated PM accumulationamount is increased and decreased in a manner as shown in FIG. 8(a),during which the estimated PM accumulation amount can be significantlydeviated from the actual accumulation amount. When the estimated PMaccumulation amount is significantly less than the actual accumulationamount, for example, when the actual accumulation amount is in a stateindicated by broken line L3 relative to the estimated PM accumulationamount indicated by solid line L1 in FIG. 7(a), the following conditionsoccur at the final stage of the PM elimination control.

That is, the burn-up control, which should be started when the estimatedPM accumulation amount (L1) is reduced to the determination value Da(0.3 g), is started in a state where the amount of PM about eachcatalyst is significantly greater (indicated by X in the FIG. 7(a)) thanthe determination value Da. Then, when the unburned fuel component issupplied to each catalyst during the control, oxidation of the unburnedfuel component causes PM to burn. However, since the actual amount of PMaccumulation is more than the assumed value (0.3 g), the heat of burningof the PM is great, which can excessively increase the catalyst bedtemperature of the NOx catalytic converter 25 and the PM filter 26.

To avoid such a problem, when execution of the PM elimination control ispossible after suspension of the control, the PM elimination control isresumed regardless of whether the PM accumulation amount is less thanthe permissible value. The resumed PM elimination control is executeduntil it is completed. FIGS. 9(a) and 9(b) show an example of changes inthe PM accumulation amount and the state of the PM elimination controlin a case where the resumption of the PM elimination control isexecuted.

In FIGS. 9(a) and 9(b), the execution of the PM elimination controlbecomes possible at time T8 after it is suspended. Then, even if the PMaccumulation amount is less than the permissible value, the PMelimination control is resumed and continued until it is completed attime T9. By completing the PM elimination control, PM accumulated aboutthe catalysts is completely burned, and the estimated PM accumulationvalue is set to zero, which eliminates an error between the estimated PMaccumulation amount and the actual accumulation amount. Therefore, theproblems described above, which are caused by the error not beingeliminated, are avoided.

The procedure for the resumption of the PM elimination control will nowbe described with reference to FIG. 10, which shows a control resumptionroutine. The control resumption routine is executed as an interrupt bythe electronic control device 50, for example, at predetermined timeintervals.

In the routine, whether the PM elimination control of the previous cyclewas suspended is determined based on a history of the operatingcondition of the engine (S101). If the outcome is positive, whether thePM elimination control is executable is determined (S102). For example,whether the engine is running and the deactivation of the catalyst iseliminated is determined (S102). Whether the deactivation of thecatalyst is eliminated is determined based on whether the catalyst bedtemperature has a value that burns unburned fuel component collected onthe catalysts (for example 300° C.) or based on whether the catalyst bedtemperature will soon have such a value because the engine load has beenhigh for a predetermined period.

When the outcome of step S102 is positive, the PM elimination control isexecuted (S103) regardless of the current PM accumulation amount, andcontinued until it is completed. Even if the PM elimination control issuspended after it has been resumed according to step S103, the PMelimination control will be continued until it is completed since thecontrol is repeatedly resumed according to steps S101 to S103.

After being resumed, the PM elimination control discretely increases thecatalyst bed temperature. The holding periods t2 and t3 for the catalystbed temperatures of 600° C. and 630° C. are shortened as the PMaccumulation amounts Dc and Db are reduced. Also, at the final stage inthe resumed PM elimination control, the burn-up control is executed tocompletely burn PM accumulated about the catalysts.

The above-described embodiment has the following advantages.

(1) When the execution of the PM elimination control becomes possibleafter it has been suspended, the PM elimination control is resumed evenif the current PM accumulation amount is less than the permissiblevalue. The resumed PM elimination control is continued until it iscompleted so that PM accumulated about the catalysts is completelyburned. When the PM is completely burned and the PM elimination controlis completed, the estimated PM accumulation value is set to zero, whicheliminates an error between the estimated PM accumulation amount and theactual accumulation amount. Therefore, repetitive execution andsuspension of the PM elimination control without completion of thecontrol are avoided. Accordingly, an error between the estimated PMaccumulation amount and the actual accumulation amount, which would beincreased during the repetitive execution and suspension, is suppressed.

(2) When the PM elimination control is suspended, burning of PMaccumulated about each catalyst has progressed to a certain degree. Theless the remaining PM accumulation at the time of suspension, theshorter the time for execution of the PM elimination control afterresumption can be set. That is, even a shorter period for execution willbe sufficient to completely burn PM accumulated about each catalyst.Accordingly, in the resumed PM elimination control, the less the PMaccumulation amounts Dc and Db, the shorter the holding periods are setfor t2 and t3. Accordingly, the execution time of the control isshortened in accordance with a decrease in the PM accumulation amount.Therefore, time required for completion of the control after resumptionis shortened. Also, degradation in the fuel consumption due to auselessly extended period for the control is avoided.

(3) In the PM elimination control resumed after suspension, the burn-upcontrol is executed at the final stage, so that PM accumulated abouteach catalyst is completely burned and the actual PM accumulation amountis reduced to zero. Therefore, when the PM elimination control iscompleted after being resumed, a situation is avoided in which PMremains about each catalyst even if the estimated PM accumulation amountis set to zero and the estimated PM accumulation amount does notcorrespond to the actual accumulation amount.

The above-described embodiments may be modified as follows.

The execution period of the PM elimination control does not need to bevaried according to the PM accumulation amount, but may be fixed for aperiod that is sufficient for PM accumulated about each catalyst to becompletely burned by the PM elimination control.

It may be configured so that the execution period is not fixed in the PMelimination control that is resumed after suspension and is fixed in thePM elimination control that is completed without being suspended.

1. An exhaust purifying apparatus for an internal combustion engine,wherein the apparatus estimates an accumulation amount of particulatematter trapped about a catalyst in an exhaust system, and wherein, whenthe estimated accumulation amount is equal to or more than a permissiblevalue, the apparatus executes PM elimination control for supplyingunburned fuel component to the catalyst to increase the temperature ofthe catalyst and burning the trapped particulate matter, and sets theestimated accumulation amount to zero at the completion of the PMelimination control, and when execution of the PM elimination controlbecomes possible after suspension of the control, the apparatus resumesthe PM elimination control even if the accumulation amount ofparticulate matter about the catalyst is less than the permissiblevalue.
 2. The exhaust purifying apparatus according to claim 1, wherein,when resuming the PM elimination control, the smaller the accumulationamount, the shorter the time for execution of the PM elimination controlis set by the apparatus.
 3. The exhaust purifying apparatus according toclaim 1, wherein, at a final stage of the PM elimination control, theapparatus executes burn-up control, in which performance and stopping ofconcentrated intermittent fuel addition to a section of the exhaustsystem that is upstream of the catalyst are repeated a predeterminednumber of times.
 4. The exhaust purifying apparatus according to claim1, wherein, when the estimated accumulation amount is less than adetermination value that is slightly more than zero, the apparatusexecutes burn-up control, in which performance and stopping ofconcentrated intermittent fuel addition to a section of the exhaustsystem that is upstream of the catalyst are repeated a predeterminednumber of times.
 5. The exhaust purifying apparatus according to ofclaim 1, wherein the apparatus discretely increases the temperature ofthe catalyst after resuming the PM elimination control.
 6. The exhaustpurifying apparatus according to claim 5, wherein the apparatus: burns,unburned fuel collected on the catalyst in an early stage of theincrease in the catalyst temperature; and further increases the catalysttemperature thereafter, thereby burning particulate matter collected onthe catalyst.
 7. An exhaust purifying method for an internal combustionengine, the method comprising: estimating an accumulation amount ofparticulate matter trapped about a catalyst in an exhaust system of theinternal combustion engine; executing PM elimination control when theestimated accumulation amount is equal to or more than a permissiblevalue, in which control, unburned fuel component is supplied to thecatalyst to increase the temperature of the catalyst and the trappedparticulate matter is burned; setting the estimated accumulation amountto zero at the completion of the PM elimination control; and resumingthe PM elimination control when execution of the PM elimination controlbecomes possible after suspension of the control, even if theaccumulation amount of particulate matter about the catalyst is lessthan the permissible value.
 8. The method according to claim 7, wherein,when the PM elimination control is resumed, the smaller the accumulationamount, the shorter the time for execution of the PM elimination controlis set.
 9. The method according to claim 7, wherein, at a final stage ofthe PM elimination control, burn-up control is executed, in whichperformance and stopping of concentrated intermittent fuel addition to asection of the exhaust system that is upstream of the catalyst arerepeated a predetermined number of times.
 10. The method according toclaim 7, wherein the temperature of the catalyst is discretely increasedafter the PM elimination control is resumed.
 11. An exhaust purifyingapparatus for an internal combustion engine, comprising: an estimationunit that estimates an accumulation amount of particulate matter trappedabout a catalyst in an exhaust system, and a control unit, wherein, whenthe estimated accumulation amount is equal to or more than a permissiblevalue, the control unit executes PM elimination control for supplyingunburned fuel component to the catalyst to increase the temperature ofthe catalyst and burning the trapped particulate matter, and sets theestimated accumulation amount to zero at the completion of the PMelimination control, and when execution of the PM elimination controlbecomes possible after suspension of the control, the control unitresumes the PM elimination control even if the accumulation amount ofparticulate matter about the catalyst is less than the permissiblevalue.